When General Sterling Price's staff decided to designate all the Arkansas infantry regiments in the District of Arkansas as "Trans-Mississippi Rifle Regiments", the 39th Arkansas Infantry Regiment was designated as the 6th Trans-Mississippi Rifle Regiment.
http://en.wikipedia.org/wiki/39th_Arkansas_Infantry_Regiment
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Re-organization of the Trans-Mississippi Department
In 1863, General Price's staff decided to designate the Arkansas infantry regiments in the District of Arkansas as Trans-Mississippi Rifle Regiments. Col. Asa S. Morgan's 26th Arkansas Regiment was designated as the 3rd Trans-Mississippi Regiment.
http://en.wikipedia.org/wiki/List_of_Arkansas_Civil_War_Confederate_units